The present invention relates to active matrix display devices and more particularly to an active matrix display device having an array of picture elements which each comprise a picture element electrode and an associated switching device and which are located at the intersections between crossing first and second sets of address conductors connected to the picture elements and via which selection and data signals respectively are supplied to the picture elements, and which includes a set of connection conductors through which selection signals are provided to the first set of address conductors, and each of which connection conductors extends in the direction of, and between, a respective adjacent pair of address conductors of the second set and is electrically connected to a respective address conductor of the first set at the location of a respective picture element.
An active matrix liquid crystal display device (AMLCD) of this kind, and suitable for use in, for example, portable applications such as mobile telephones, camera viewfinders, electronic personal organisers and the like, is described in European Patent Application No. 01200466.9 (PHNL 010074).
The provision of the set of connection conductors enables addressing signals for driving the picture elements, comprising selection (scanning) signals applied to the row address conductors and display data signals applied to the column address conductors, to be supplied at either a common side of the support or at opposing, parallel, sides of the support rather than at two mutually perpendicular sides as in conventional AMLCDs and the like. In a conventional AMLCD a set of row address conductors, carrying the selection signals, and a set of column address conductors, carrying the data signals, each extend over a rectangular support beyond the area of the array of picture elements electrodes to respective peripheral regions of the support on which they are carried, and comprising two adjacent, edge parts of the support, for the purpose of enabling electrical contact to be made with the sets of address conductors. For example, row and column drive circuit ICs may be directly mounted on these peripheral border regions of the support with their output terminals connected to the extended address conductors or, alternatively, may be mounted on foil with their output terminals connected to the address conductors via tracks on the foil. The set of connection conductors allows the ICs to be provided instead either at a common peripheral border region along just the side of the support or at respective peripheral border regions along opposing, parallel, sides of the support, or alternatively for foil connections to be made at such parts.
As described in the aforementioned application, this feature can be used, for example, to enable the effective display area for a given size of support to be increased in one dimension, which is of benefit when the display device is used in small portable products. A similar kind of connection scheme is described in the paper by R. Greene et al entitled xe2x80x9cManufacturing of Large Wide-View Angle Seamless Tiled AMLCDs for Business and Consumer Applicationsxe2x80x9d, IDMC 2000, pages 191-194. The benefit in this case is that facilitate tiling of individual display panels is facilitated by allowing the address conductors to be driven from just one edge.
It has been found, however, that the operational characteristics of such display devices can be affected adversely by the presence of the connection conductors. In particular, it has been found that the display device can suffer picture element non-uniformity and image retention problems.
It is an object of the present invention to provide an improved display device of the kind described in the opening paragraph.
It is a further objection of the present invention to provide a display device in which the aforementioned problems are alleviated at least to some extent.
According to the present invention, there is provided an active matrix display device of the kind described in the opening paragraph wherein in each picture element a parasitic capacitance exists between the picture element electrode and a connection conductor, and wherein picture elements other than those at the locations of the connections between the connection conductors and the address conductors of the first set are each provided with a supplementary capacitance between their picture element electrodes and their associated address conductors of the first set.
The supplementary capacitances lead to improved display quality with unwanted display artefacts being reduced significantly. The invention stems from a recognition that certain parasitic capacitance effects are introduced by the presence of the connection conductors which are responsible for the aforementioned problems. The nature of the parasitic capacitances means that those picture elements which are situated at the locations of the connections between the connection conductors and their associated address conductors of the first set, and which are coupled to the address conductors connected at those points to the connection conductors, experience different consequences to picture elements elsewhere in the array. As a result then so-called kickback effects for picture elements at the connection point locations are unlike those for other picture elements and for a similar given applied data signal voltage, therefore, their display outputs will be different to display outputs from the other picture elements. Moreover, these picture elements can be affected by a consequential rms DC voltage producing ageing effects in the LC material and an image retention problem. By adding a capacitance of suitably selected value to the other picture elements then the kickback effects for all picture elements can be made similar, thereby improving picture element uniformity. The value of the supplementary capacitance may be approximately equal to the parasitic capacitance between the picture element electrode and a connection conductor.
Typically in active matrix display devices such as AMLCDs, the picture elements inevitably have a parasitic capacitance present between their picture element electrode and their associated address conductor carrying the selection signals, the value of which capacitance can vary according to the exact nature of its structure. The added capacitance will be additional to this particular parasitic capacitance. Such a parasitic capacitance can result from an edge of the picture element electrode extending alongside, or perhaps slightly overlapping, a portion of the address conductor and separated therefrom by dielectric material. The added capacitance may be provided by altering these contributory parts in picture elements other than those at the locations of the connection points in order to increase the parasitic capacitance. This could, for example, be achieved by locally thinning intervening dielectric material in the case of overlying picture element electrodes and address conductor portions. Preferably, however, it is achieved conveniently by providing or increasing an area of overlap between the electrode and the address conductor, either by designing a part of the electrode to extend over the address conductor to a greater extent or vice versa. Alternatively, the added capacitance may be provided in the form of a separate capacitor structure connected between the picture element electrode and the address conductor.
The connection conductors may extend as lines beneath the picture element electrodes, and be separated therefrom by an intervening layer of dielectric material. Alternatively, these lines may be arranged to extend laterally adjacent the picture element electrodes, for example alongside the address conductors of the second set. This would reduce the capacitance between these conductors and the picture element electrodes which is an important factor as the value of the capacitance added to picture elements needed would then be much smaller. This can be beneficial also in the case of the display device being a transmissive type with the picture element electrodes comprising a transparent conductive material, such as ITO, as otherwise the lines would similarly need to be of transparent material when placed underneath the picture element electrodes. However, positioning these lines laterally adjacent the picture element electrodes could result in a decrease in picture element aperture.
Although applicable particularly to active matrix liquid crystal displays devices, the invention may advantageously be applied also to different kinds of active matrix display devices using other electro-optic materials for the picture elements, such as electrophoretic, and electrochromic display devices.